Day 6 – How Many Galaxies Are Out There?

Imagine if our world had a diary and on the interesting days, someone pulled out a big book and wrote about it.

Day 6. By this stage in the life of the Universe (11,500,000,000 years ago) there exists something of the order of 170,000,000,000 (170 billion) galaxies each containing between tens of millions and billions of stars.

The name ‘galaxy’ is from the Greek word for milk which is how they described the whiteish appearance of the band of light we know as our home galaxy. Each of the 170 billion galaxies is indeed a group of stars, but much more than that. They almost certainly hold billions of planets, star systems, star clusters and in most cases, a massive black hole near the centre. They also contain a lot of material in the form of vast clouds of gas and dust particles that are mostly the remains of stars that have exploded at the end of their lives.
It has become clear that every galaxy has more mass (the ‘weight’ of something on earth) than we can measure by adding up what we can see. (A cannon ball in space may be weightless but it would still make a mess if it hit you because it still has the same mass.) This non-reflective material is predictably called ‘dark matter’ which incorporates ‘dark energy’. As Einstein showed, matter and energy are different forms of the same thing.

Galaxies come in a range of sizes and types from the dwarf galaxies with only 10 million stars to the big boys that can have one hundred trillion (that is, a hundred million million). 100,000,000,000,000. That’s a lot of zeros and it’s hard to imagine a number so large, especially when we are talking about very hot objects the size of our Sun, which itself is a million times bigger than the earth.
In between there’s not a lot going on, but technically, intergalactic space is not empty, but just as near to it as all get out, maybe 1 atom per cubic metre, which is sparse by anyone’s standards. In the popular press, quoting distances is done in light years, however the scientists in this field tend to use the parsec, which is 3.26156 light years. One might be forgiven for thinking that it’s not exactly a round number, an easily remembered number or one that trips lightly off the tongue, so why use it?
When we see a star, at the same time we also see stars further away. Six months later, when we are on the opposite side of our orbit around the sun, that particular star will appear to have moved slightly compared to the background stars. That’s because we are looking from a different angle.
You can close one eye and look at something close, then look through the other eye and it seems to move against the background. The parsec number is defined by measuring this angle. If it appears to move by 1/3600th of a degree, that is one parsec (like 3,600 seconds in an hour) the light would take 3.26156 years to reach us. The cosmologists need really big numbers and even light years aren’t big enough for the job, so parsecs it is.
Over the years, a fair number of galaxies have been catalogued, tens of thousands in fact but here is the interesting bit, there are 5 main catalogues and they don’t have all the same galaxies listed. Even when they are, they have different identification numbers.

Take Messier 109 for example. In the Messier catalogue, as you would expect, it’s number 109 but in the other catalogues it’s also code number NCG3992, UGC6937, CGCG 269-023, MCG +09-20-044 and PGC 37617. Clever boys.
Now, in the rest of the scientific world, the custom is to assign a name to whatever is being studied, even the least significant, invisible to us, microbe. Here we have entire galaxies, billions of times larger than our whole planet and all they get is a number. Well, up to five numbers actually, depending on how many catalogues we are looking at.

It should therefore come as no surprise that someone thought the galaxies were not getting a fair deal and decided to make a new catalogue, called with great imagination, ‘The Catalogue of Named Galaxies’ to give them a bit of dignity and some spiffy names.
Belgian astrophysicist Gerard Bodifee and the classicist Michel Berger began their new catalogue with one thousand well-known galaxies. Each were given a meaningful and descriptive name using the methods of other sciences like biology, palaeontology and anatomy. For example the aforementioned Messier 109 became the new ‘Callimorphus Ursae Majoris’.
Hell yeah, that’s a much better name.

One cannot pursue the subject of galaxies without mentioning ring galaxies, unusual in that they don’t seem to have much in the middle. They are thought to form in a type of bull’s eye collision when a smaller galaxy passes through the middle, cleaning up so to speak, leaving a neat ring of stars behind.
It looks like that might be what happened to the Andromeda Galaxy, perhaps more than once as it displays a multi-ring structure. There won’t be many rings left after the next collision when our own massive Milky Way smacks into it head on, which it is scheduled to do soon. (Soon in galactic terms, about 200,000,000 years from now.)

Despite the prominence of our galaxy and the millions of other large and very large galaxies, in sheer numbers, dwarfs rule. In fact you could say most galaxies are the smaller variety, perhaps only one hundredth the size of our Milky Way and often host to fewer than a hundred million stars.
Some of these are so small, light photons only take 326 years to travel from one side to the other, almost like crossing the street in galactic terms. They have all the same shapes as their larger cousins, elliptical, spiral and irregular, although the dwarf ellipticals require a little imagination to classify them that way.

Checking out our neighbourhood reveals that twenty seven of the local galaxies are dwarfs and that in itself is somehow surprising, but better yet, the mass of each dwarf (equal to about ten million of our Sun) seems to be very similar, regardless of the number of stars. This, it is argued, lends weight if you pardon the pun, to the conjecture that dark energy/matter is indeed the dark horse in gravitational theory.

AND this is just a one line entry in what I believe is the greatest scientific gift poster that anyone could gift to a child. But that’s just my opinion.